Processes for removal of oxides from the surface of metals



Patented Apr. 27, 1954 UNITED STATES OFFICE PROCESSES FOR REMOVAL OFOXIDES FROM THE SURFACE OF METALS No Drawing. Application July 8, 1947,Serial No. 759,712

Claims. 1

Our invention relates to a process for removal of dense, firmly adherentoxide formed on the surface of metals, through exposure to air, underhigh pressures, at elevated temperatures, by immersion of the metal in afused anhydrous bath of caustic alkali containing an oxidizing agent, toconvert the oxide to a form in which it is more reactive with acids, andthen subjecting the modified oxide to the action of a dilute acid, asdescribed and claimed in U. S. Patent No. 2,395,694, now re-issued asPatent No. Re. 22,887.

The process of the patents is stated therein to be useful for removal ofsurface oxides from iron, nickel and cobalt and their alloys with eachother and with chromium, manganese, molybdenum, tungsten, vanadium andcopper. The caustic alkalies suitable for the process are stated toinclude sodium and potassium hydroxide, and the oxidizing agents toinclude the alkali metal and alkaline earth chlorates, nitrates,chromates, manganates and permanganates.

In the above described process the efliciency of the fused bath is afunction of its oxidizing capacity. With continued use of the bath itsoxidizing capacity diminishes. It must therefore be restored in orderthat the bath be kept at high efliciency. Heretofore, the only method ofdoing so has been by addition of a further quantity of oxidizing agent.

The present invention has for its object to regenerate the oxidizingagent in situ, thus decreasing the consumption of oxidizing agent andincreasing the capacity of a given equipment, in some cases with anactual improvement in the cleaning of the metal. 7

The oxidizing agents disclosed in the patents referred to may be dividedinto two groups as follows: (a) The chlorates, which in giving up theiravailable oxygen are reduced to chlorides, which cannot be regenerated,so that substantial addition of oxidizing agent from time to time isnecessary, in order that the bath may be constantly maintained at a highefficiency; and (1)) Those, such as nitrates, manganates, permangantes,chromates, etc., which after reduction to a lower state of oxidation canbe reoxidized to their former state. The oxidizing agents of group (b)which have become reduced are slowly reoxidized by the oxygen of the airwhen the bath is not in use. However, during the use of the bath at highoutput, the atmospheric reoxidation is too slow to keep pace with thereduction and the efficiency of the bath therefore falls off.

- We have now found that the regeneration of the oxidizing agents ofgroup (b) can be accelerated by supplying elemental oxygen to the bath,as by bubbling air therethrough, either intermittently or while the bathis in use. The quantity of air may be quite small, e. g. 0.1 cubic footper minute per ton of material in the bath. Contrary to expectation,this does not greatly increase the carbonate content of the bath.

We have also found that the regeneration of the oxidizing agent byexposure to the atmosphere is likewise accelerated by addition to thebath of a minor proportion, based on the oxidizing agent, of an oxygencarrier, i. e., a compound of a metal having a higher oxidationpotential than that of the reduced oxidizing agent, hence capable ofgiving up oxygen thereto, and susceptible of atmospheric oxidation froma lower to a higher state of oxidation. This compound may be an oxide ora salt metal such as manganese, chromium, vanadium or molybdenum; or itmay be a compound in which the metal occurs in the acid radical.

We have likewise found that if air is bubbled through the bathcontaining such an oxygen carrier the regeneration is still furtheraccelerated and the capacity and effectiveness of the bath are therebyvery notably increased.

Example I For regeneration of the bath of caustic soda containing 4.07per cent of sodium nitrite by oxidation of the nitrite to nitratethrough contact with the atmosphere an exposure of 90 hours at 500 C.has been found necessary. If, however, air is bubbled through the bath,the time required for regeneration is decreased to hours. If 0.5 percent MnOz is added to the bath the time required for regeneration of thenitrate by exposure to the atmosphere at 500 C. is decreased to 11hours; but if air is bubbled through the bath to which M1102 has beenadded, the time necessary for regeneration of the bath is furtherdecreased to two hours.

Example I shows the manganese is highly effective as a carrier of oxygento sodium nitrite, for the purpose of our process.

Example II We have similarly found that regeneration of a bath ofcaustic soda containing 4.25 per cent sodium chromite, by oxidation ofthe chromite to 5 per cent chromate by exposure to the atmos,

in twenty hours by blowing air therethrough in the absence of manganeseor in 5 hours by air blowing in presence of the manganese.

Example II shows that manganese is also effective as a carrier of oxygento sodium chromite.

Example III The following table affords an approximate comparison of therelative efficiency of various oxygen carriers as compared with MnOz,for accelerating regeneration of 4.07 per cent NaNOz to 5 per cent NaNOaat 500 0., without air blow- Time of Regeneration, hours Percent andKind of Oxygen Carrier ther shows that while vanadium and molybdenum arelikewise effective, chromium, which is always present in baths used fortreating chrome steels, is poor, and iron, which is always present inbaths used for treating ferrous metals is negligible, as an oxygencarrier.

It should be noted that the proportion of 0.5 per cent of oxygencarrier, given in the above table, which is calculated as the oxide, isnot the minimum proportion necessary to yield a useful result, whichdepends on the particular oxygen carrier used and also upon whether itis assisted by air blowing. We believe the practical minimum to be 0.25per cent without air blowing and 0.1 per cent if air blowing is used.

We are aware that it has been proposed in U. 5. Patent 2,271,37 to add0.5 to per cent manganese dioxide or copper sulphate to a fused bath ofcaustic a-lkali containing an alkali metal nitrate, phosphate orcarbonate, for the purpose of increasing the'fiuidity thereof; but thispatent does not disclose that the nitrate becomes reduced to nitriteduring use of the bath, that this slowly oxidizes back to nitratethrough contact with the atmosphere; that such reoxidation isaccelerated by presence of manganese or that the reoxidation is stillfurther greatly accelerated by air blowing in presence of manganese.

It will be noted that among the oxidizing agents listed above assuitable for the process of the patents are the chromates andpermanganates, which are also listed as oxygen carriers suitable for ourpresent purpose. However, Example III shows that these are not allequally effective. The oxygen carrier should be in minor proportion withrespect to the oxidizing agent, and preferably in the proportion of 0.5to 1.0 per cent. The oxidizing agent and oxygen carrier may in fact beinterchangeable, the latter being merely a minor addition of a differentoxidizing agent. The present invention therefore contemplates, in. thecarrying out of the process of the patents referred to: (a) bubblingoxygen, as such or as air, through the bath of fused caustic alkalicontaining an oxidizing agent susceptible of oxidation from a lower to ahigher state of oxidation and an oxygen carrier capable of giving upoxygen to the reduced oxidizing agent and susceptible of oxidation froma lower to a higher state of oxidation, whether normally present or not,and (b) adding to such a bath such an oxygen carrier, not normallypresent in the bath, or in proportion greater than normally present, andcontacting the bath with oxygen, either by exposure to the atmosphere orby bubbling the air therethrough.

Any suitable method and means may be used for introducing the air intothe molten bath to be regenerated, as for example, bubbling the airbelow the surface of the bath through an iron pipe, preferably in such aWay as to cause circulation of the bath.

In the process of the patents referred to, the oxidizing agent ispreferably in minor proportion, e. g. 1 to 20 per cent, and preferably 5to 10 per cent; however, we have found that the present process isparticularly useful when the fused bath contains a relatively highproportion of oxidizing agent, e. g. 50 per cent or more, for the reasonthat baths containing such large proportions of oxidizing agentsregenerate by surface exposure to air more reluctantly than when theoxidizing agent is present in minor proportion.

We claim as our invention:

1. In the removal of dense firmly adherent oxide from the surface ofmetals including the step of subjecting the oxide to the action of afused substantially anhydrous bath of caustic alkali containing anoxidizing agent stable therewith under the conditions of the process andsusceptible of regeneration by oxidation from a lower to a higher stateof oxidation, and a minor proportion, based on the oxidizing agent, ofmaterial susceptible of oxidation to a state of oxidation in which it iscapable of giving up oxygen to the reduced oxidizing agent, the methodof accelerating the regeneration of the oxidizing agent, which comprisesbubbling oxygen through the bath.

2. In the removal of dense firmly adherent oxide from the surface ofmetals including the step of subjecting the oxide to the action of afused substantially anhydrous bath of caustic alkali containing anOXldlZlIlg agent of the group consisting of the alkali metal andalkaline earth nitrates and chromates, and a minor proportion. based onthe oxidizing agent, of an oxygen carrier of the group consisting of thealkali metal and alkaline earth compounds of manganese, vanadium, andmolybdenum, the method of accelerating the regeneration of the oxidizingagent which comprises bubbling oxygen through the bath.

'3. In the removal of dense, firmly adherent oxide from the surface ofmetals of the group consisting of iron, nickel and cobalt and otheralloys with each other and with chromium, molybdenum, tungsten, vanadiumand copper, including the step of subjecting the oxide to the action ofa fused substantially anhydrous bath of caustic alkali containing notless than 1 per cent of sodium nitrate and 0.1 to 1.0 per cent of acompound of manganese stable with the bath, under the conditions of theprocess, calculated as manganese dioxide, the method of accelerating theregeneration of the oxidizing agent which comprises bubbling oxygenthrough the bath.

4.. In the removal of dense firmly adherent oxide from the surface ofmetals, including the step of subjecting the oxide to the :action of afused substantially anhydrous bath of caustic alkali containing anoxidizing agent stable therewith under the conditions of the process andsusceptible of regeneration by oxidation from a lower to a higher stateof oxidation, the method of acceleratin the regeneration of theoxidizing agent which comprises adding to the bath a minor proportion,based on the oxidizing agent, of a material susceptible of oxidation toa state of oxidation in which it is capable of giving up oxygen to thereduced oxidizing agent, and contacting the bath with air below thesurface thereof to supply oxygen to the bath in excess over the normalexposure of the bath surface to the atmosphere.

5. In the removal of dense firmly adherent oxide from the surface ofmetals of the groupconsisting of iron, nickel, and cobalt and theiralloys with each other and with chromium, manganese, molybdenum,tungsten, vanadium, and copper, including the step of subjecting theoxide to the action of a fused substantially anhydrous bath of causticalkali containing an oxidizing agent of the group consisting of thealkali metal and alkaline earth nitrates, chromates, manganates andpermanganates, the method of accelerating the regeneration of theoxidizing agent which comprises adding to the bath a minor proportion,based on the oxidizing agent, of a compound of a metal other than thoseundergoing treatment and agent, susceptible of oxidation from a lower toa higher state of oxidation and in the latter state capable of giving upoxygen to the reduced oxidizing agent, and contacting the bath with airbelow the surface thereof to supply oxygen to the bath in excess overthe normal exposure of the bath surface to the atmosphere.

6. In the removal of dense firmly adherent oxide from the surface ofmetals of the group consisting of iron, nickel, cobalt, and their alloyswith each other and with chromium, manganese, molybdenum, tungsten,vanadium and copper, including the step of subjecting the oxide to theaction of a fused substantially anhydrous bath of caustic alkali,containing an oxidizing agent of the group consisting of the alkalimetal and alkaline earth nitrates, chromates, manganates andpermanganates, the method of accelerating the regeneration of theoxidizing agent which comprises adding to the bath a materialsusceptible of oxidation to a state in which it is capable of giving upoxygen to the reduced oxidizing agent, in proportion minor with respectto the oxidizing agent, but substantially greater than normally presentin the bath, and contacting the bath with air below the surface thereofto supply oxygen to the bath in excess over the normal exposure of thebath surface to the atmosphere.

7. In the removal of dense firmly adherent oxide from the surface ofmetals of the group consisting of iron, nickel, and cobalt and theiralloys with each other and with chromium, molybdenum, tungsten, vanadiumand copper, including the step of subjecting the oxide to the action ofa fused substantially anhydrous bath of caustic alkali containing anoxidizing agent of the group consisting of the alkali metal and alkalineearth nitrates and chromates, the method of accelerating theregeneration of the oxidizing agent which comprises adding to the bath aminor proportion, based on the oxidizing agent, of an oxygen carrier ofthe group consisting of a compound of manganese stable with the bathunder the conditions of the process, and conin the oxidizing 6 tactingthe bath with air below the surface thereof to supply oxygen to the bathin excess over the normal exposure of the bath surface to theatmosphere.

8. In the removal of dense firmly adherent oxide from the surface ofmetals of the group consisting of iron, nickel, and cobalt and theiralloys with each other and with chromium, molybdenum, tungsten, vanadiumand copper, including the step of subjecting the oxide to the action ofa fused substantially anhydrous bath of caustic alkali containing notless than 1 per cent of a reduced oxidizing agent of the groupconsisting of the alkali metal and alkaline earth nitrates andchromates, the method of acceleratin the regeneration of the oxidizingagent which comprises adding to the bath 0.25 to 1.0 per cent of acompound of manganese stable with the bath under the conditions of theprocess, calculated as manganese dioxide, and contacting the bath withair below the surface thereof to supply oxygen to the bath in excessover the normal exposure of the bath surface to the atmosphere.

9. In the removal of dense firmly adherent oxide from the surface ofmetals, including the step of subjecting the oxide to the action of afused substantially anhydrous bath of caustic alkali exposed toatmospheric oxygen and containing an oxidizing agent stable with thebath under the conditions of the process and susceptible of regenerationby oxidation from a lower to a higher state of oxidation, the method ofaccelerating the regeneration of the oxidizing agent Which comprisesmaintaining in the bath a minor proportion, based on the oxidizingagent, of an oxygen carrier susceptible of oxidation to a state ofoxidation in which it is capable of giving up oxygen to the reducedoxidizing agent at a rate eiiecting regeneration corresponding to anincrease from approximately four percent NaN02 to approximately fivepercent NaNOs at a temperature of 500' degrees centigrade in not morethan 20 hours, said carrier being selected from the group consisting ofMnOz, M11504, M003, V205, C103, and mixtures thereof.

10. The method of claim 9 wherein the oxidizing agent is present inminor proportion and wherein the oxygen carrier is present in the bathin about 0.25 to 1.0 percent by weight based on the oxidizing agent.

11. The method of claim 9 wherein the oxygen carrier is Mn02.

12. The method of claim 9 wherein the oxygen carrier is MnSOr.

13. The method of claim 9 wherein the oxygen carrier is M003.

14. The method of claim 9 wherein the oxygen carrier is V205.

15. The method of claim 9 wherein the oxygen carrier is CrOa.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Re. 22,887 Spence June 3, 1947 2,337,062 Page Dec. 21, 19432,395,694 Spence Feb. 26, 1946 OTHER REFERENCES Mellor: ComprehensiveTreatise on Inorganic and Theoretical Chemistry, vol. 8, p. 4'77,Longmans, Green & 00., New York, 1928. (Copy in Div. 59.)

1. IN THE REMOVAL OF DENSE FIRMLY ADHERENT OXIDE FORM THE SURFACE OFMETALS INCLUDING THE STEP OF SUBJECTING THE OXIDE TO THE ACTION OF AFUSED SUBSTANTIALLY ANHYDROUS BATH OF CAUSTIC ALKALI CONTAINING ANDOXIDIZING AGENT STABLE THEREWITH UNDER THE CONDITIONS OF THE PROCESS ANDSUSCEPTIBLE OF REGENERATION BY OXIDATION FROM A LOWER TO A HIGHER STATEOF OXIDATION, AND A MINOR PROPORTION, BASED ON THE OXIDIZING AGENT, OF AMATERIAL SUSCEPTIBLE OF OXIDATION TO A STATE OF OXIDATION IN WHICH IT ISCAPABLE TO GIVING UP OXYGEN TO THE REDUCED OXIDIZING AGENT, THE METHODOF ACCELERATING THE REGENERATION OF THE OXIDIZING AGENT, WHICH COMPRISESBUBBLING OXYGEN THROUGH THE BATH.